In a wide range of industrial processes, it is frequently necessary to apply a coating to an article. In some cases, where the coating serves as a protective barrier, uniformity of the coating is of paramount importance. In many electrical devices, the application of a conductive layer also requires the appropriate density or thickness minimums be met as well as uniformity in the coating of the surface such as a printed circuit board as any non-uniformity can lead to a failure in the circuit and a complete elimination of the article as they are typically not salvageable or repairable at a reasonable cost. Thus, while a coating in an electrically conductive device need not be highly uniform it is required that in the intended patterns the coating be continuous to assure a closed circuit.
Where the surface or surfaces to be coated have unusual geometries, the prior art has attempted a variety of techniques to achieve-the desired uniformity of the coating as well as continuity of the coating over the required portions of the article. While some of these methods have had limited success, with the increasingly complex geometries encountered, the prior art techniques have not been able to achieve a high degree of uniformity at a reasonable cost both in terms of the time required to effect coating as well as the expense of the techniques employed.
In a specific coating application involving multi-layer printed circuit boards, through holes having a high aspect ratio are required to enable the inter-leaved circuit layers to be connected together. In more recent designs, the number of layers employed has been increasing to enable greater capacity and/or complexity in terms of the operations of the equipment that will incorporate the board. It has been recognized that the improved uniformity of the coating in the through holes in these types of applications presents a significant obstacle to increased production efficiency and capacity in view of the great amount of time required in conventional electroplating systems to effect uniform coating of the surfaces of the through holes.
In an attempt to solve this difficulty, the prior art such as is represented by U.S. Pat. No. 4,875,982, granted Oct. 24, 1989, has proposed a time-consuming and, therefore expensive, technique of specifically addressing each hole formed in a board for the purpose of improving flow through the hole of a solution including a plating element such as copper ions in a copper sulfate bath. Other iS attempts have been directed to electrochemical reactions but these require a pre-coating of the board with an agent that will effectively react with a solution containing a metal to be deposited. In this regard, reference may be had to U.S. Pat. No. 4,891,069, granted Jan. 2nd, 1990. Reverse electric pulse plating has been employed where the electric current is alternated for selected intervals. This has improved the plating process generally but has not proven particularly useful with through or blind holes in boards.